Light enables us to interact with the outside world. It is the form of energy that brings information about the surroundings, and about the cosmos. In everyday life we often complain about its excess or lack. In some cases we have to be careful because light can be dangerous. In order to understand these dangers and protect oneself against them, one must know the basic principles of the interaction between light energy and matter in general.

Light is electromagnetic radiation that has various properties: intensity, spectrum, coherence, polarization, divergence. The difficulty to understand its effect comes from the fact that we do not have direct access to these properties. We are aware of the intensity and colors, but the fact that it is a wave with quantum properties is not obvious. In fact, only since the end of the 19th century the first complete theory about the classical physical aspects of light has been available. It has been completed by the description of quantum properties in the beginning of the 20th century.

This article describes the basic properties of light in relation to hazards to the human body. Danger is great during the use of coherent light sources, the lasers. But natural light also contains dangers against which it is quite easy to protect oneself.